ADPCM synthesizer without look-up table

ABSTRACT

The present invention provides a circuit for use in an ADPCM synthesizer for generating a function of a quantization step without the use of a look-up table. Illustratively, the function is represented in a piecewise linear fashion. This permits an ADPCM synthesizer to be implemented without the use of a memory storing a look-up table for the function so the area of the synthesizer chip can be reduced.

FIELD OF THE INVENTION

The present invention relates to an Adaptive Differential Pulse Code Modulation (ADPCM) synthesizer which does not require the use of a look-up table.

BACKGROUND OF THE INVENTION

In signal processing applications such as speech, data is compressed before it is stored and decompressed when it is retrieved. Such compression/decompression reduces the amount of memory required to store the data. One of the most common compression/decompression techniques is ADPCM.

When ADPCM is used, the analysis (compression) process is as follows:

1. Consider the discrete data samples, X₀, X₁, X₂, . . . X_(N-1). It should be noted that this data has maximum points.

2. Select an initial estimated value X₀ and an initial quantization step q_(ini) ;

3. From the original data value X₀ and the initial estimated data value X₀, a quantization step size q₀ and compressed data value D₀ can be obtained as follows:

    q.sub.0 =f.sub.1 (X.sub.0, X.sub.0, q.sub.ini)

    D.sub.0 =f.sub.2 (X.sub.0, X.sub.0, q.sub.ini)

4. From the obtained values q₀, D₀, the next estimated value,

    X.sub.1 =X.sub.0 +T(q.sub.0)+D.sub.0

is obtained, where T is a function of q (quantization step). The function T is usually an exponential function and, conventionally, a look-up table stored in a memory is used to implement this function. The function T(q) is illustrated in FIG. 1.

5. The values q_(n), D_(n), X_(n) can be calculated as follows:

    q.sub.n =f.sub.1 (X.sub.n-1, X.sub.n-1, q.sub.n-1)

    D.sub.n =f.sub.2 (X.sub.n-1, X.sub.n-1, q.sub.n-1)

    X.sub.n =X.sub.n-1 +T(q.sub.n-1)*D.sub.n-1

6. In this way, the original data X₀, X₁, X₂, . . . , X_(N-1) are transformed into estimated data X₀, X₁, X₂, . . . , X_(N-1) and the encoded (compressed) data D₀, D₁, D₂, . . . , D_(N-1) are also obtained.

Generally, when ADPCM is used, the above-described analysis process is carried out in software using a computer to generate the compressed data D₀, D₁, D₂, . . . , D_(N-1).

When ADPCM is used, the synthesis (decompression) process is as follows:

1. Select the same initial quantization step size value q_(ini) and estimated value X₀.

2. From the compressed data D₀, D₁, . . . , D_(N), the next estimated value X₁, X₂, . . . and quantization step q₀, q₁, . . . can be calculated as follows:

    X.sub.n+1 =X.sub.n +T(q.sub.n)*D.sub.n

    q.sub.n+1 =f.sub.3 (q.sub.n, D.sub.n)

Using this formulation, the sequence of estimated values X₀, X₁, X₂, . . . , X_(N) is regenerated from the compressed data.

FIG. 2 schematically illustrates a circuit 10 for performing the ADPCM synthesis process described above. In the circuit 10 of FIG. 2, the input is the compressed data D_(n). The compressed data D_(n) is received at the input 11. The circuit 12 receives the inputs D_(n), q_(n) at the inputs 13, 14 respectively. The output of the circuit 12 is q_(n+1) =f₃ (q_(n), D_(n)) which appears at the output 15. The output q_(n+1) is then processed by the unit delay 16 so that q_(n) is present at the input 13. The unit delay 16 also transmits the value q_(n) to a memory circuit 20 which stores a look-up table for the function T(q_(n)). FIG. 3 is an exemplary look-up table for the function T(q_(n)) and thus illustrates the values stored in the memory 20. The circuit 22 multiplies T(q_(n)) and D_(n) and outputs the product T(q_(n))*D_(n). The product T(q_(n))*D_(n) is one input to the summer 24. The other input is X_(n). The value X_(n) is generated by the unit delay 26. The output of the summer 24 is X_(n+1) which is equal to X_(n) +T(q_(n))*D_(n). The value X_(n+1) is fed back to the unit delay 26 whose output is X_(n).

The values contained in the look-up table stored in the memory 20 of FIG. 2 and explicitly set forth in FIG. 3 are illustrative only and are obtained according to the formula:

    q.sub.n =integer part of q.sub.n-1 *1.1

where

q_(n) =16.

The memory 20 of FIG. 2 is shown in greater detail in FIG. 4. As shown in FIG. 4, the memory 20 has six input pins for the 63 steps q_(n) =0 to q_(n) =62 and twelve output pins for the output values T(q_(n)). Thus, the memory 20 requires 63×12=756 bits. This memory cannot be easily divided into smaller blocks and, thus, results in a significant increase in the chip size for the ADPCM synthesizer.

Accordingly, it is an object of the present invention to provide an ADPCM synthesizer which does not utilize a memory storing a look-up table to implement the function T(q_(n)).

SUMMARY OF THE INVENTION

To carry out this objective, the present invention provides a circuit which implements the function T(q_(n)) without use of a look-up table stored in a memory.

In accordance with the present invention, the function T(q_(n)) is represented in a circuit in a piecewise linear fashion. Illustratively, the input q_(n) has six bits q₅ q₄ q₃ q₂ q₁ q₀. The basic output value is 1q₂ q₁ q₀ 0. This basic output value is shifted a number of bits to the left to generate the final output T(q_(n)) depending on the value q₅ q₄ q₃. For example, if q_(n) =8=001000, then the basic value=1q₂ q₁ q₀ 0=10000=16. The number of shifts to the left is given by q₅ q₄ q₃ =001. Thus, the output value T(q_(n))=100000 =32 is obtained by shifting the basic value one place to the left.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 schematically illustrates the function T(q_(n)).

FIG. 2 schematically illustrates a prior art ADPCM synthesizer.

FIG. 3 illustrates values comprising a look-up table for the function T(q_(n)) stored in a memory in the synthesizer circuit of FIG. 2.

FIG. 4 illustrates a memory which stores the look-up table of FIG. 3.

FIG. 5 is a piecewise linear representation of the function T(q_(n)).

FIG. 6 is a circuit which implements a piecewise linear representation of the function T(q_(n)) in accordance with the present invention.

FIG. 7 is a table which relates inputs and outputs for the circuit of FIG. 6.

FIG. 8 is an ADPCM synthesizer which utilizes the circuit of FIG. 6.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with the present invention, the exponential function T(q_(n)) is represented in a piecewise linear manner. A piecewise linear representation T'(q_(n)) of an exponential function T(q_(n)) is illustrated in FIG. 5. Thus, the function T(q_(n)) illustrated in the look-up table of FIG. 3 is approximated by

    T'(0)=16

    T'(q.sub.n)=(16+(q.sub.n -8*(q.sub.n /8))*2)*2.sup.(q.sbsp.n.sup./8)

where

(q_(n) /8) is the integer part of q_(n) divided by eight.

For example, if

    for q.sub.n =1, T'(1)=(16+(1-8*0)*2)*2.sup.0 =18

    for q.sub.n =8, T'(8)=(16+(8-8*1)*2)*2.sup.1 =32

FIG. 6 is a circuit 50 for implementing the piecewise linear function T'(q_(n)). The quantization step size q_(n) is illustratively represented by six bits, i.e., q_(n) =q₅ q₄ q₃ q₂ q₁ q₀. The circuit 50 has two sets of inputs. The first set of inputs 52 receives the bits q₂ q₁ q₀. The second set of inputs 54 receives the bits q₅ q₄ q₃. The outputs 56 comprise twelve bit positions of the values T'(q_(n)). To generate the outputs 56 from the inputs 52, 54, the circuit 50 comprises a plurality of processing elements in the form of MOS transistors 58. The input 52 above q₂ (i.e., the input 52') is set to logic "1" and the remainder of the inputs 52 (besides the inputs receiving q₂, q₁, q₀) are set to logic "0".

In the circuit 50, the transistors 58 are arranged in three columns 62, 64, 66. The bit values q₃, q₄, q₅ are applied to the gates of the transistors in the columns 62, 64, 66, either directly or via corresponding inverters 63, 65, 67. The bit values q₀, q₁, q₂ are connected to the drain terminals of particular transistors in the column 66. The source terminals of the transistors in the column 66 are connected to the drain terminals of particular transistors in the column 64. The source terminals of the transistors in the column 64 are connected to the drain terminals of particular transistors in the column 62. The source terminals of the transistors in the column 62 are connected to corresponding outputs 56.

The circuit 50 operates as follows:

At the inputs 52, there are present the values 00000001q₂ q₁ q₀ 0 which are obtained from the bits q₂ q₁ q₀ of the quantization step q_(n). The basic value 1q₂ q₁ q₀ 0 is shifted to the left a particular number of places depending on the values q₅ q₄ q₃ as follows:

When q₅ =0, the output data is the same as the input data, i.e., 00000001q₂ q₁ q₀ 0, when q₅ =1, the input data is shifted four places to the left so that the output data is 0001q₂ q₁ q₀ 00000

When q₅ =0, and q₄ =0, the output data is unchanged from the input, i.e., 00000001_(q) ₂ q₁ q₀ 0

When q₅ =1, and q₄ =0, the input data is shifted four places to the left so the output data is 0001q₂ q₁ q₀ 00000

When q₅ =0, and q₄ =1, the input data is shifted two places to the left so that the output data is 000001q₂ q₁ q₀ 000

When q₅ =1, and q₄ =1, the input data is shifted six places to the left so that the output data is 01q₂ q₁ q₀ 0000000

When q₃ is equal to 1, there is an additional shift of one place to the left.

FIG. 7 is a table which summarizes the relationship between q₅ q₄ q₃ and the output data.

FIG. 8 illustrates an ADPCM circuit 10' in accordance with the present invention. In FIG. 7, the circuit 50 of FIG. 5 replaces the memory table 20 in the prior art ADPCM synthesizer of FIG. 2. This reduces the chip area in the ADPCM synthesizer.

Finally, the above described embodiments of the invention are intended to be illustrative only. Numerous alternative embodiments may be devised without departing from the spirit and scope of the following claims. 

The claimed invention is:
 1. A circuit for use in an adaptive differential pulse code modulation synthesizer for implementing a function T'(q_(n)) of a previously determined quantization step q_(n) which is represented by a plurality binary bits comprising:a first set of inputs for receiving a first group of said binary bits of said previously determined quantization step q_(n) ; a second set of inputs for receiving a second group of said binary bits of said determined quantization step q_(n) ; and a plurality of processing elements for processing said first and second groups of binary bits to generate a binary output representing said function T'(q_(n)) having a basic value formed from said first group of binary bits, wherein said basic value is shifted by a number of places determined by said second group of binary bits; wherein said binary output representing said function T'(q_(n)) is multiplied in a multiplier with a compressed data value D_(n) and the product T'(q_(n))*D_(n) thus produced is summed in a summer with a previously determined estimated decompressed data value X_(n) to obtain a next decompressed data value X_(n+1).
 2. The circuit of claim 1 wherein said quantization step q_(n) is represented by the binary bits q₅ q₄ q₃ q₂ q₁ q₀, said basic value is given by 1q₂ q₁ q₀ 0, and the number of places the basic value is shifted is given by q₅ q₄ q₃.
 3. The circuit of claim 1 wherein said function of said quantization step is a piecewise linear function.
 4. An adaptive differential pulse code modulation synthesizer circuit comprising:a first circuit which receives a previously determined quantization step q_(n) and which outputs a function T'(q_(n)), said first circuit representing said function T'(q_(n)) in a piecewise linear fashion, a second circuit which multiplies T'(q_(n)) and a compressed data value D_(n) to obtain a product T'(q_(n))*D_(n), and a summer which sums a previously determined estimated decompressed data value X_(n) and T'(q_(n))*D_(n) to obtain a next estimated decompressed data value X_(n+1) wherein said quantization step q_(n) is represented by a plurality of binary bits and said first circuit comprises:a first set of inputs for receiving a first group of said binary bits, a second set of inputs for receiving a second group of said binary bits, and a plurality of processing elements for processing said first and second groups of binary bits to generate a binary output having a basic value formed from said first set of binary bits, wherein said basic value is shifted by a number of places determined by said second set of binary bits.
 5. The synthesizer circuit of claim 4 wherein said synthesizer circuit includes circuit means for obtaining a next quantization step q_(n+1) as a function of said previously determined quantization step q_(n) and said compressed data value D_(n).
 6. An adaptive differential pulse code modulation synthesizer circuit comprising:a first circuit for receiving a previously determined quantization step q_(n) and for outputting a function T'(q_(n)), which is a piecewise linear implementation of an exponential function T(q_(n)), without the use of a look-up table storing values of said function T'(q_(n)), a second circuit for multiplying T'(q_(n)) and a compressed data value D_(n), and a summer for summing the product T'(q_(n))*D_(n) outputted by said second circuit and a previously determined estimated compressed data value X_(n) to obtain a next decompressed data value X_(n+1). 